This invention generally relates to methods for removing aluminosilicate-type materials from various substrates. In some specific embodiments, the aluminosilicate material is in the form of a deposit that accumulates on turbine engine components. For example, the aluminosilicate material being removed may reside on airfoil surfaces, or within internal cooling passages.
Ceramic coatings are often used to thermally insulate various sections of turbine engine components, such as the combustor. The coatings allow the engine to operate more efficiently at high temperatures. Examples of such coatings are the thermal barrier coatings (TBC's), which are often zirconia-based, and stabilized with a material like yttria Such coatings must have low thermal conductivity, strongly adhere to the component, and remain adherent throughout many heating and cooling cycles.
The TBC's are often held tightly to the substrate with a metallic bond coating. The bond coatings usually belong to one of two classes: diffusion coatings or overlay coatings. State-of-the-art diffusion coatings are generally formed of aluminide-type alloys, such as nickel-aluminide, platinum-aluminide, or nickel-platinum-aluminide. Overlay coatings typically have the composition MCrAlY, where M is Ni, Co, Fe, or some combination thereof.
In view of the high temperature and harsh operating conditions to which they are sometimes exposed, the TBC's sometimes need to be repaired or replaced. As described in U.S. Pat. No. 6,379,749 (Zimmerman, Jr., et al), a variety of circumstances may require removal of the TBC. Examples include damage during engine operation; coating defects; handling damage, and the like.
Some of the state-of-the-art methods for repairing components protected by a TBC result in removal of the entire TBC system, i.e., both the ceramic coating, as well as the underlying bond coat. The two coatings usually must then be re-deposited. Moreover, techniques used to remove the coatings, such as grit-blasting, can be slow and labor-intensive. These techniques can also be difficult to control, and can sometimes damage the substrate surface beneath the bond coat. With repetitive use, these procedures may eventually destroy the component by reducing its wall thickness.
Other potential problems occur when the bond coat is a diffusion coating. For example, a diffusion aluminide-type coating (e.g., platinum-aluminide) includes a diffusion zone that extends into the substrate surface of the component. Damage to this type of bond coat can occur by the fracturing of brittle phases in the diffusion zone or the overlying additive zone. Furthermore, repeated stripping and re-applications of diffusion-aluminide coatings can undesirably alter the thickness of the component.
As a response to these problems, nonabrasive processes have been developed for removing a TBC. For example, an autoclaving process is sometimes used, in which the TBC is subjected to elevated temperatures and pressures, in the presence of a caustic compound. Another process involves the use of a halogen-containing powder or gas, such as ammonium fluoride (NH4F).
A particularly effective process for removing a ceramic coating is described in the above-mentioned U.S. Pat. No. 6,379,749. In that process, the TBC is treated with an aqueous solution which contains an acid fluoride such as ammonium bifluoride (NH4HF2), along with a corrosion inhibitor. The process efficiently removes TBC material from various surfaces of the component, without damaging the substrate, or any bond coat which may be present. (After removal of the TBC, the bond coat can be quickly rejuvenated by known techniques, to restore its oxidation protection). Moreover, the process is effective for removing the TBC from any cavities in the component, such as the cooling holes usually present in turbine airfoils.
The ammonium bifluoride process has many advantages in removing ceramic coatings from various surfaces. However, the process is sometimes rendered ineffective in the presence of dirt which may reside on the ceramic surfaces. In the case of turbine engines, the dirt is often formed as various engine deposits during high-speed operation. It is sometimes referred to as “CMAS” (calcium-magnesium-aluminosilicate). In addition to impeding the effectiveness of the ammonium bifluoride solution, CMAS (initially in molten form) can infiltrate and damage the TBC on a turbine engine component. Moreover, CMAS, in fine, particulate form, can also become trapped in various cooling passages within the component. The presence of the CMAS in these regions can undesirably reduce cooling efficiency.
It should thus be apparent that processes for efficiently removing aluminosilicate material from various substrates would be welcome in the art. The processes should also be capable of removing the aluminosilicate material from cavities within the substrate, e.g., cooling passageways. Moreover, these new cleaning techniques should not adversely affect the substrate. They should also not adversely affect any protective coating applied thereon, if the coating is meant to be retained. The processes should also be free of any unacceptable amounts of hazardous fumes in the workplace, or any effluent which cannot easily be treated. Furthermore, these treatment processes should be compatible with other treatment techniques being employed, e.g., stripping processes for removing TBC's and/or bond coat materials.